Set explicit return types for `"inngest/fastify"`

[jpwilliams]

Summary

Address "slow types" for "inngest/fastify" handler.

Checklist

• [ ] ~Added a docs PR that references this PR~ N/A
• [ ] ~Added unit/integration tests~ N/A
• [x] Added changesets if applicable

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 6849d3155a96b87261872d7c907cc35f63d6cce7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------- | ----- | | inngest | Patch |

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